Redundancy is widely used in various process controls which need a higher reliability. Input/output (I/O) redundancy in a discrete controlling system (DCS) provides two modules with identical hardware and firmware structures, namely, one is an active module, and the other is a passive or backup module. As soon as an error is happened in the active module, a switchover from the active module to the passive module will be initiated. Then, an active module will stop working, and the passive module will take over the responsibility to keep the process control running. Redundancy design in the I/O module increases the reliability of the DCS and make sure that an error in the I/O module will not affect the process control.
There are two prior art solutions for analog output module redundancy. First is so called parallel redundancy. In this solution, both the active and passive modules output 50% of the power respectively and work in parallel. However, the parallel redundancy requires a high reliability in module synchronization as well as accuracy of each single module. Therefore, the parallel redundancy is not only difficult to implement, but also its manufacturing cost will be relatively higher.
Another prior art solution is to make the active module to output 100% of the power and the passive module to only establish an output circle inside the pass module itself. This solution is simpler and the cost is lower. In this solution, since the output circle is already established inside the passive module, the switchover is also bumpless. However, an output current inside the passive module also increases power consumption of the passive module. And power consumption of the redundant passive module might be a big problem for system power dissipation and also not good for minimizing a size of the module.
FIG. 1 schematically illustrates a redundant circuit structure according to the prior art. As shown in FIG. 1, the circuit structure comprises an active module 12 and a passive module 14. In a normal situation, a first switch 122 in the active module is opened, an output voltage 124 in the active module 12 powers a load 16 via a current control device 126, for example, a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET). Meantime, a second switch 142 in the passive module 14 is closed to make sure the passive module 14 do not power the load 16. In other words, in the normal situation, the active module 12 provides a power to the load 16, and the passive module 14 is in a backup state. In a case that an error is happened in the active module 12, a switchover from the active module 12 to the passive module 14 will be initiated. In this time, the first switch 122 is closed, and the second switch 142 in the passive module 14 is opened. Thus, the voltage output 144 of the passive module 14 powers the load 16 via a current control device 146, for example, MOSFET. However, this solution makes the power consumption of the passive module quite high because the higher voltage output 144 is still applied to the second current control device (MOSFET) 146 even though the passive module 14 is in the backup state, and therefore not good for heat dissipation.